1. Field of the Invention
The present invention relates to products having compositions incorporating rubber crumbs derived primarily by recycling rubber tires and, more particularly, to the manufacture of precast underfoot surfacing, containing recycled tire rubber and other rubber scrap for walkways, driveways and the like.
2. The Prior Art
An ongoing critical need exists for an environmentally compatible paving and outdoor flooring overlay that is resistant to wear, weather and chemicals. The present invention contemplates the use, as a substantial ingredient in such overlays, of vulcanized rubber crumbs derived from the ever increasing supply of scrap rubber tires. The present invention is environmentally compatible in the sense that it utilizes a growing accumulation of scrap that is very difficult to discard, in a product that is very resistant to deterioration.
The problem of scrap tires and their disposal has occupied the public's attention of many for years. Some have suggested using whole scrap tires for erecting structures. See Martin Pawley, Building for Tomorrow: Putting Waste to Work, Sierra Club Books, San Francisco, 1982. Others have developed machines designed to cut the tread portion from the tire for use as a roof covering. Ibid. Still others have turned to incineration for the direct production of energy, or to tire rubber chips as a fuel supplement in paper mills, cement kilns and other industrial heat generators. For one reason or another, few of these recycling efforts have been adequately successful, commercially or environmentally.
Recycled rubber, as defined by the Rubber Recycling Division of the National Association of Recycling Industries, Inc., generally refers to any sort of rubber waste, including scrap tires, that has been converted into an economically useful form, such as ground rubber, reprocessed rubber, and die-cut punched rubber parts. In contrast to recycled rubber, reclaimed rubber represents a product resulting from a process in which waste vulcanized scrap rubber is treated to produce a plastic-like, devulcanized material that can be processed, compounded and re-vulcanized with or without the addition of either natural or synthetic binders. The vulcanization process technically, by definition, is irreversible under normal conditions. Nevertheless, an accepted definition for "devulcanization" is a change in vulcanized condition which results in decreased resistance to deformation at ordinary temperatures.
Most techniques for reclaiming scrap rubber involve continuous batch processing. In one conventional continuous technique, scrap tire rubber is ground, and any foreign components such as metal and/or fiber are mechanically separated from the ground rubber remainder, which then is further ground to finer particle size. Next, the finely ground rubber, together with various reclaiming agents, are metered into a blending system and conveyed to a special screw-extrusion machine. In this machine, the finely ground rubber is softened by a controlled and variable amount of high heat and pressure in a continuously moving environment. The softened rubber matrix next is continuously discharged through an extrusion head, whereupon it is cooled and conveyed to the millroom for the final stage of the reclaiming process. For screw extruders, see D. H. Morton-Jones, Polymer Processing, Chapman & Hall, 1989. In the millroom, the softened and now cooled rubber is generally mixed with ingredients in a suitable blender, then reheated and replasticized, i.e. devulcanized, in a barrel-mixer. Thence it is fed first to a high-friction breaker mill and next to a high-friction refiner mill. A high-friction ratio for the refiner mill is achieved with different-size rolls rotated at considerably different speeds. The rolls are set tightly to produce a thin sheet of reclaimed rubber that is smooth, uniform and free of grains and lumps. The finished thin sheet of reclaimed rubber emanating from the refiner mill is pulled to a wind-up drum, and allowed to build up to a thickness of about an inch before being cut therefrom by a knife. The resulting reclaimed rubber is in the form of an intermediate slab, which is dusted to reduce tackiness, and stacked. Since reclaimed rubber is cheaper than natural or synthetic rubber, it is widely used in the manufacture of new rubber goods, including new tires.